A light beam incident to an anisotropic medium changes into two light beams after being refracted, which is called birefringence. The two refracted beams are linearly polarized light with vibration directions orthogonal to each other, one beam follows the law of refraction, which is called ordinary light (o light); and the other beam does not follow the law of refraction, which is called extraordinary light (e light). Due to the anisotropy of the crystalline materials, the size of the angle between the two refracted beams is related to the direction of propagation of light and polarization state of light. The birefringent crystals can be divided into uniaxial and biaxial crystals, wherein uniaxial crystal materials belong to a trigonal, tetragonal or hexagonal system, while biaxial crystal materials belong to a triclinic, monoclinic or orthorhombic system, and the birefringent materials convenient in use are uniaxial crystals. Birefringence of the crystal is an important optical parameter of optic-electric functional materials. Birefringent crystal materials have important applications in the fields of optics and communications, and widely used to fabricate polarizer prisms and polarizing beam splitter prisms and so on. In the following descriptions, “a” and “c” stand for unit cell parameters of a trigonal system; Z stands for the number of molecules in the unit cell; “no ” and “ne ” stand for refractive indices for the ordinary wave and extraordinary wave respectively.
The commonly used birefringent crystal materials are mainly MgF2 crystal, quartz crystal, YVO4 crystal, calcite (CaCO3) crystal, and crystals such as α-BBO crystal, Ca3(BO3)2 crystal and ReBa3B9O18 (Re=Y, Pr, Nd, Sm, Eu, Gd, Tb, Er, Dy, Ho, Tm, Yb, Lu) crystal which have been reported in recent years. The MgF2 crystal has a transmission range of 110-8500 nm, which is an excellent material applied in DUV range; however, due to its low birefringence, the MgF2 crystal is not suitable to fabricate Glan polarizers, but only suitable for Rochon polarizers; moreover, the light separation angle of the MgF2 crystal is small and the size of the fabricated device is usually large, which limits its application. The quartz crystal is also disadvantaged in small birefringence, causing the same problem as MgF2 crystal. The YVO4 crystal is an excellent artificial birefringent crystal, however, its transmission range is 400-5000 nm, preventing it from being applied in the UV range. The calcite crystal with large birefringence is the most commonly used birefringent crystal, however, as it mainly exists in the form of natural minerals which usually contain high level of impurities, it is difficult to obtain the calcite crystal in ultraviolet optical level; devices based on the calcite crystal can normally be used for the wavelength above 350 nm, and applications in DUV (below 250 nm) cannot be achieved. In recent years, several borate birefringent crystals have been reported. The high temperature phase BaB2O4 (α-BBO) crystal has a transmission range of 189-3500 nm and a large birefringence, however, α-BBO crystal is easy to deliquesce and there exists solid-state phase transition in the crystal, making it easy to crack during crystal growth and thereby affecting the yield and availability of the crystal; besides, UV transmittance cut-off wavelength of the α-BBO crystal is only 189 nm, which limits its application in wavelength range below 200 nm. The Ca3(BO3)2 crystal has a transmission range of 180-3800 nm, however, it is disadvantaged in small birefringence in the visible region and low transmittance in the deep UV region, which also limits its use. Due to containing rare earth elements, the YBa3B9O18 crystal has absorption in the ultraviolet region, with transmittance being 50% at 300 nm and decreasing to 10% at 220 nm, and therefore is not suitable for use in the DUV range. The Ba2Mg(B3O6)2 birefringent crystal of the prevent invention has a wide transmittance range (177-3000 nm) and large birefringence (no-nc=0.077-0.229), and can be applied in ultraviolet (UV) or deep ultraviolet (DUV) range (180-350 nm).